In a known tool change apparatus of that kind (German published specification (DE-AS) No. 16 52 699), two change arms are provided in diametrally opposite relationship on a common pivotal shaft. At its end, each of the change arms has a semicylindrical bearing shell, the inside diameter of which approximately corresponds to the outside diameter of the cylindrical portion. A locking bar which is movable substantially in the longitudinal direction of the change arm is arranged on the change arm, opposite the bearing shell. For the purposes of the tool change operation, the change arm is pivoted into the region of the tool holder receiving means, and partially embraces, with its semicylindrical shell, the cylindrical portion of the tool which is to be replaced. When that occurs, the locking bar is retracted against the force of a spring and, when the semicylindrical shell bears against the cylindrical portion, presses against the cylindrical portion at a location which is in opposite relationship to the semicylindrical shell. However, that kind of holding arrangement is not sufficiently precise if the tools to be changed are those whose tool holder fits with a very close fit into the tool holder receiving means.
In addition, the change arm is of comparatively large structural dimensions in the region of the semicylindrical shell so that the tools must be arranged at a substantial spacing from each other in a tool magazine into which they are transferred by the change arm. Consequently, the tool magazine is of comparatively large dimensions. So that the tool holder which is to be exchanged occupies, in its axial direction, a given position relative to the change arm and is not unintentionally displaced relative to the tool arm in the change operation, it is also necessary for shoulders to be provided in front of and behind the cylindrical portion, in the axial direction, with the diameter of the shoulders being smaller than the diameter of the cylindrical portion. Accordingly, on both sides of the semicylindrical shell, the change arm has portions which are also somewhat smaller in diameter than the diameter of the semicylindrical shell. The shoulders cause the tool holder to be weakened in its cross-section or, if a weakening in cross-section is to be avoided, the semicylindrical portion must be made of correspondingly larger diameter, which in turn makes it necessary for the tool holders to be arranged at even larger spacings from each other in the tool magazine.
The most usual type of tool change apparatuses operates with tongs-like grippers which are respectively provided at the end of the change arm. The tool holder has a flange in which a groove of trapezoidal cross-section is turned. The tongs of the gripper engage into the groove. In this construction also the connection between the gripping tongs members and the tool holder is comparatively lacking in accuracy. Furthermore, the tongs members and also the flange which is required suffer from the disadvantage which has already been mentioned above, namely that the tool holders must be arranged at comparatively large spacings from each other in the tool magazine.
The invention is therefore based on the problem of providing a tool change apparatus for a machine tool of the kind set forth in the opening part of this specification, which, while avoiding the above-mentioned disadvantages, permits a connection between the tool holder and the change arm to be made with a very high degree of positional accuracy, which is of a simple construction, which is of small dimensions even in the region in particular of the change head and wherein the tool holder does not need to have any portions of enlarged diameter for engagement of the change arm, so that the tool holders can be disposed in a tool magazine at minimum spacings from each other.
According to the invention, that problem is solved in that provided in the cylindrical portion is a radially extending screwthreaded bore, that provided at the free end of the change arm is a change head which is movable radially with respect to the axis of the cylindrical portion and which is provided with a prism-shaped centering recess at its outer end which is towards the tool holder, and that provided in the change head is a connecting pin which extends in the centre of the centering recess in the radial direction of movement of the change head and which can be driven by motor means in two opposite directions of rotation and which is limitedly displaceable in its axial direction in the change head and which is provided at its free end with a male screwthread which fits into the screwthreaded bore.
The tool change apparatus according to the invention permits the connection made between the tool holder and the change head to have a very high degree of positional accuracy for the two components are fixedly screwed together by means of the connecting pin in the too change operation. In that respect, the positional accuracy as between the two components is also decisively ensured by the prism-shaped recess in the change head.
There is also the advantage that the tool holder only has to have the cylindrical portion and does not have to have any further portions which project beyond the diameter of the cylindrical portion or also portions which are of smaller diameter, for engagement of the change arm. As moreover the change head engages the tool holder only from one side, there is no need to provide space for the change arm or parts thereof, laterally beside the tool holder, when fitting the tool holder into a tool magazine or into the tool holder receiving means. Consequently, the tool holders can be disposed at very small spacings from each other in the magazine, and the pitch or partitioning of the magazine only needs to be immaterially larger than the diameter of the cylindrical portion.
In addition, the tool holder is not weakened by any grooves or shoulders, to that the tool holder is extremely strong. By virtue of the connection between the change head and the tool holder providing a high degree of positional accuracy, it is also readily possible to change tool holders which are fitted into a tool holder receiving means, with a very close fit. In addition, the novel tool change apparatus can be used for all systems for making a connection between the tool holder and the tool holder receiving means.